Exhaust gas recirculation (EGR) is a technology used with internal combustion engines to reduce nitric oxides (NOx) in the engine exhaust. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. The recirculated exhaust, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature. In a spark ignited engine, the recirculated exhaust dilutes the amount of combustible matter in the cylinder.
A properly operating EGR system can increase the efficiency of a spark ignited engine. EGR improves the knock tolerance of the engine, which allows improved combustion phasing and increased compression ratio. The combined effect of the improved phasing and compression ratio improves efficiency and reduces fuel consumption.
EGR has conventionally been utilized as inert dilution. A conventional spark ignited engine's EGR system typically sources the EGR from the same exhaust volume that is delivered to the emissions control aftertreatment device. Therefore, the EGR gas composition is governed by the emission control device requirements for inlet gas composition.
However, it may be advantageous to decouple the EGR from the constraints of the exhaust aftertreatment system. Changing the EGR composition and quantity have been shown to significantly improve combustion on spark ignited engines using EGR. Decoupling the exhaust EGR from the aftertreatment gas constraints can be achieved to enable higher quality EGR while still maintaining exhaust composition compliant with conventional three-way catalyst after treatment.